1. Field of the Invention
The invention relates to a cold cathode ionization manometer for measuring pressure in a vacuum, operating in accordance with the inverse magnetron principle and used for detecting pressures in the fine, high and ultra-high vacuum range (preferably between 107 Pa and 1 Pa).
2. The Prior Art.
In ionization manometers of this type a spontaneous cold gas discharge is ignited by a high voltage between two unheated electrodes—the anode and the cathode—suitably arranged in the housing of a measuring tube. The ignition is maintained up to the smallest pressures by an additional magnetic field of sufficient field strength. The discharge current which flows during this time is dependent on the pressure over a very broad vacuum range and (usually) serves as a measure of the pressure. Known structures of cold cathode ionization manometers currently used commercially are the electrode configurations proposed by Penning and the magnetron and inverse magnetron tube structure developed by Redhead. A detailed description of such manometers can be found in Chr. Edelmann's book “Vakuumphysik”, Spektrum Akad. Verl. Berlin, (1998).
Aside from their wide measuring range, cold cathode ionization manometers are characterized not only by a simple structure, they are also sturdy and immune to air irruptions and vibrations and they are relatively inexpensive. In addition to the numerous advantages to be derived from the use of these manometers, these measuring instruments nevertheless suffer from the decisive disadvantage which at least in time significantly limits their usefulness. With increasing service life, their measuring accuracy diminishes in consequence of contaminations of their measuring cells. These contaminations stem from the gas discharges taking place in the cold cathode ionization manometers. Since the charge carrier concentration in the measuring cells increases with increasing pressure thus leading to the generation of ever-increasing discharge currents, in the high and particularly in the fine vacuum range the high electric fields cause more and more ions to be accelerated strongly in the direction of the cathode and, upon impact, pulverize the cathode material. This sputtered cathode material is deposited on the walls and on electric insulating paths within the measuring tube and may there form conductive layers. An applied operating voltage may lead to leakage currents which are superimposed upon the discharge current and thus significantly limit the lower measuring range of the manometer. As a result of the described cathode pulverization, the useful life of the cathode is of limited duration. Since as a rule the cathode material is stainless steel, released particles of the formed contamination layers may be aligned in the magnetic field and lead to short circuits. Moreover, in the pressure range above 10−2 Pa, hydrocarbon containing vapors (oil vapors, etc.) may be cracked or polymerized in the plasma by interaction with high-energy charge carriers. The cracking products (usually layers of carbon) formed by these plasma-chemical reactions may also be deposited on electrode surfaces and there form surface layers which adversely affect the yield of their secondary electrons and which may shift characteristic curves and render an unstable indication of pressure. In the least favorable condition it is even possible that the discharge is extinguished with an accompanying failure of the measuring tube.
Since as a result of the interference effects described supra the useful life of the measuring instruments is limited, all conventional commercial cold cathode ionization manometers are structured such that they may be disassembled easily for cleaning contaminated electrodes or for replacing them by new ones. In this fashion it is possible completely to restore the original operating condition and, hence, efficiency of the measuring tube even after extended use, by removal of the contaminating layers from the measuring tube. However, in order lastingly to ensure full efficiency of these vacuum gages, it is necessary to calibrate or clean the measuring tubes at regular intervals which always entails increased maintenance and costs.
Although the effect of the contamination of these measuring tubes and their consequences has been known from the time of their application in vacuum measuring technology there are, or have been, few, if any, promising approaches to its reduction. Thus, a Penning gage is known from German patent specification 197 17 263 the cathode of which consists at least primarily of titanium. In view of the low yield of sputtered titanium, the pulverizing of the cathode material should proceed slowly which should result in an extended useful life of the cathode. It should, however, be taken into consideration that of all materials, titanium, because of its property of chemically absorbing active gases, is used, for instance, as cathode material in (Ti-) atomizing pumps, so that it is to be expected that Penning tubes equipped with Ti cathodes are subject to substantially higher pumping action than are cold cathode measuring tubes provided with conventional cathode material. Hence, the pressure indicated by them are likely to suffer from greater errors.
An earlier patent specification, GB 555,134, is based upon the concept of automatically reducing the operating voltage of the measuring tube by an electronic control circuit when, at a defined pressure in the upper vacuum range, the discharge current has attained a predetermined threshold value. If the pressure is increased further, the anode voltage is electronically reduced such that the discharge current remains constant, and the controlled anode voltage now serves as a measure of the pressure. Initially, this method was proposed as a method of broadening the measuring range of cold cathode ionizing manometers towards higher pressures. However, it may be assumed that it would also contribute to reducing the effect of contamination in the upper pressure range since owing to the lowering of the operating voltage the electrical field strengths are lowered as well, so that ions impinge upon the surface of the cathode at reduced energy. Moreover, by lowering the operating voltage, the discharge currents become smaller. The two actions, i.e. lowering the operating voltage and reducing the discharge current, result in a diminished rate of contamination. A similar but substantially simpler method is based upon examinations by Conn and Daglish (“The Influence of the Ballast Resistance on the Performance of Penning Vacuum Gages”; J. Sci. Instr. 31 (1954); pp. 403–434) which for limiting the discharge current at high pressures inserted a high-ohmic resistance between the anode of the measuring tube and the high voltage apparatus. As a result of this so-called ballast resistance the operating voltage at the anode is reduced as the pressure increases (and, hence, at increasing discharge current) so that the discharge current is limited at sufficiently high pressures.
In accordance with another patent specification, U.S. Pat. No. 4,000,457, the contamination of a tube may be avoided by applying to the measuring tube, at the upper vacuum range, a direct voltage pulsed as a function of pressure. While in the ultra-high and high vacuum ranges the cathode measuring tube is operated by a direct voltage in the conventional manner, a sinusoidal alternating voltage is superposed on the operating voltage as a function of the discharge current increasing with increasing pressure. In the upper pressure range the direct voltage component is lowered sufficiently that the discharge at high pressures is now pulsed sinusoidally only. As a result of this pulse measuring tube energization, the susceptibility to contamination of cold cathode measuring tubes is retarded, so that a substantially longer service life may be expected.